The invention has been developed especially in connection with the problems that are encountered in cases of fractures of the wrist or lowermost in the radius, one of the bones of the forearm, a very common fracture. Usually, fairly good results are achieved with plaster treatment, but it is known that in 10-20% of cases this results in an unacceptable malalignment or malunion, especially fore-shortening. If there is presumed to be a danger of such malalignment or malunion, it is known to use external fixation. Transcutaneous screws are then screwed into the radius above the fracture and into the second metacarpal bone, and the alignment is locked by means of lockable universal joints, rods etc. At least two screws are used in each bone. When this technique is employed, the fracture lowermost in the radius is fixed indirectly because both the wrist joint and the joints in the campus are between the bones which are fixed with the screws. Despite this, this known method has proven to be adequate, and healing with a good alignment is usually achieved. However, the joints become stiff, and osteoporosis is a common complication. This causes considerable problems of rehabilitation and often a lasting reduction of mobility in wrist and finger joints, and there is therefore a certain degree of scepticism with respect to this method in the medical profession.
The drawbacks outlined above can be avoided if a distraction apparatus or fixator is used which permits the patient to move the joints during treatment.
Various fixators or distraction apparatus are known which seek to achieve this. For instance, No. 170 513 makes known an external bone fixation device which consists of fixation rods connected to bone fixation pins capable of being inserted into the bone, and an articulation part which joins the two rods. This known fixation device enables the patient to carry out some mobility exercises with the broken joint. DE 43 13 767 makes known a fixation device which includes a hinge member allowing limited joint movements.
U.S. Pat. No. 4,628,919 makes known an external fixation device which includes two articulatedly connected rods, attached to respective bone pins. This known device allows a certain freedom of movement in the wrist joint.
U.S. Pat. No. 5,437,666 makes known an external fixation device for osteosynthesis, which device includes bone fixation pins and connecting rods having a central disk articulation, and the device enables motion of the wrist joint about three axes.
Similarly, U.S. Pat. No. 5,437,667 makes known a fixation device for a distal fracture of the radius. Here too, a hinge mechanism is used which allows motion that does not differ from the normal kinematic motion of the wrist.
U.S. Pat. No. 5,047,865 makes known a distraction device where two bone fixation pins are interconnected by means of two springs which act as a compression spring and an extension spring, respectively. The device is especially intended for intra-articular fracture dislocations of fingers.